Air filter



FIG. 1 is a perspective view showing the new design for an air filter;

FIG. 2 is a top view thereof;

FIG. 3 is a bottom view thereof;

FIG. 4 is a front view thereof; and,

FIG. 5 is a side view thereof.

The features shown in broken lines depict environmental structure only and form no part of the claimed design. 

CLAIM The ornamental design for an air filter, as shown and described. 